1. Field of the Invention
This invention relates to an aqueous coal slurry composition. More particularly, it relates to an aqueous slurry composition which is excellent in the dispersion stability and has such an excellent static stability that even if the composition is allowed to stand still for a long time, a hard cake of a dense and compact precipitate is not formed.
2. Description of the Prior Art
Petroleum has heretofore been used in largest quantities as the energy source, but because of limited oil deposits and attendant increase of the price of petroleum, it has recently been desired to use a variety of energy sources and maintain stable supply thereof. Under such circumstance, effective utilization of coal which is present all over the world with large quantities of deposits has been reconsidered. However, coal is solid unlike petroleum and impossible to transport through pipelines, and thus, handling of coal is disadvantageous. Furthermore, since coal contains a much larger amount of ash than petroleum, such trobules as reduction of the calorific value and disposal of fly ash arise. In order to eliminate the disadvantages in handling coal, various researches have been made on the method in which coal is powdered and dispersed in water and the resulting aqueous slurry is handled and used. However, this aqueous coal slurry is still not satisfactory in that if the coal concentration is increased, the viscosity is drastically increased and the fluidity is poor and, in contrast, if the coal concentration is reduced, the transportation efficiency is reduced and the dehydration step becomes expensive. Furthermore, it is difficult to find an optimum coal concentration. Namely, agglegation of coal particles is caused in an aqueous coal slurry to increase the viscosity and reduce the fluidity. As the size of coal particles in the aqueous slurry is smaller, the dispersion stability is better, but the pulverization cost increases with elevation of the degree of fine pulverization. Fine coal now used in thermal power plants has such a particle size that 80% of particles pass through a 200-mesh sieve, that is, a particle size of about 74 microns. Accordingly, it is expected that this particle size is used as one standard value of the particle size of fine coal.
When a surface active agent which is a dispersant is added to an aqueous coal slurry, the surface active agent is adsorbed in the interface between coal particles and water to exert functions of disentangling coal particles and prevent coal particles from agglegation. Accordingly, it is expected that addition of the surface active agent will produce a good dispersion state. We already proposed as such a dispersant a sulfonation product of a polycyclic aromatic compound which may contain a hydrocarbon group as a substituent or its salt (see Japanese Unexamined Patent Publication No. 21,636/81). When this dispersant is used, the fluidity is improved, but this dispersant is practically not satisfactory in that when a slurry containing this dispersant is allowed to stand still for a long time, a precipitate is formed and this precipitate becomes dense and compact to form a hard cake.